Diagnostic equipment for an exhaust gas cleaning apparatus

ABSTRACT

A diagnostic equipment for an exhaust gas cleaning apparatus installed for an engine, comprising a misfire detector which detects the misfire of the engine, and a secondary-air-system failure detector which detects the failure of a secondary air system. An index corrector corrects a deterioration index calculated by a deterioration-index calculator, in accordance with the detected result of the detector. A deterioration decision unit decides if the diagnostic equipment has failed, by the use of the corrected deterioration index. In a case where the extent of the misfire or the like is severe, a decision interrupter interrupts the decision of the deterioration decision unit. Thus, even when the misfire of the engine or the failure of the secondary air system has occurred, the detection of the deterioration of a catalyst does not err. It is therefore avoided to erroneously replace the catalyst which has not deteriorated yet, or to run the engine in spite of the deterioration of the catalyst.

This application is a division of application Ser. No. 08/795,142, filedFeb. 7, 1997, which is a continuation of application Ser. No.08/224,881, filed Apr. 8, 1994, now U.S. Pat. No. 5,743,085. Thisdocument claims priority to U.S. Provisional Application Ser. No.05/083,326 filed Apr. 9, 1993, now U.S. Pat. No. 3,705,700.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diagnostic equipment for an exhaustgas cleaning apparatus installed in an engine system. More particularly,it relates to a diagnostic equipment for an engine exhaust gas cleaningapparatus which employs a catalyst together with a so-called called"UEGO sensor(universal exhaust gas oxygen sensor)" that serves tomeasure an air fuel ratio in a wide range of air-fuel-ratio values or aso-called "O₂ -sensor" (oxygen sensor) that generates a binary output onthe basis of a sudden output change near a stoichiometric ratio(hereinbelow, both the sensors shall be collectively called an"air-fuel-ratio sensor").

2. Description of the Related Art

A well-known apparatus for cleaning the exhaust gas of an engine has acatalyst and an air-fuel-ratio feedback controller. The catalyst isincorporated in an exhaust pipe part for the purpose of eliminating HC(hydrocarbons), NO_(x) (nitrogen oxides) and CO (carbon monoxide) whichare contained in the exhaust gas, The air-fuel-ratio feedback controlleris a device which is disposed for the purpose of causing the catalyst todemonstrate the function thereof satisfactorily, and which executes acontrol so as to hold the air fuel ratio of the exhaust gas at apredetermined value, while measuring the air fuel ratio by the use of anair-fuel-ratio sensor mounted upstream of the catalyst.

In such an exhaust gas cleaning apparatus, when the air-fuel-ratiosensor for measuring the air fuel ratio has undergone deterioration inits performance, the air fuel ratio sometimes deviates from thepredetermined value, resulting in increase in the amounts of the noxiousgas components contained in the exhaust gas. Moreover, the air fuelratio may fall outside a ratio range in which the catalyst candemonstrate its performance, and the elimination efficiency (conversionefficiency) of the catalyst for the noxious gases may lower. On theother hand, when the catalyst itself has undergone deterioration in itsperformance, the conversion efficiency thereof for the noxious gaseslowers in spite of the control of the air fuel ratio into the ratiorange in which the catalyst can demonstrate its performance. In thismanner, the deterioration of the performance of the air-fuel-ratiosensor or the catalyst results in increasing the amounts of the noxiousgases which are emitted into the atmosphere. Therefore, a diagnosticequipment for the exhaust gas cleaning apparatus has been contrived inorder to diagnose the performances during the drive of a vehiclefurnished with the cleaning apparatus and to give warning to the driverof the vehicle against the deteriorations. By way of example, thediagnostic equipment is so constructed and operated that air-fuel-ratiosensors are respectively disposed upstream and downstream of thecatalyst, and that an air-fuel-ratio feedback control is executed on thebasis of, at least, the output of the air-fuel-ratio sensor locatedupstream of the catalyst, while the deterioration of the catalyst isdetected on the basis of the output of the air-fuel-ratio sensor locateddownstream of the catalyst, etc. Such prior-art techniques are disclosedin the official gazettes of Japanese Patent Applications Laid-open No.91440/1990 and No. 286160/1991.

With the method wherein, during the air-fuel-ratio feedback control, thedeterioration of the catalyst is detected on the basis of the output ofthe air-fuel-ratio sensor located downstream of the catalyst, naturallythe detection of the deterioration of the catalyst is impossible whilethe air-fuel-ratio feedback control is at rest. Besides, in such a casewhere the catalyst has not been activated yet, the detection of thecatalyst deterioration is highly liable to err. It is thereforenecessary to permit and inhibit the detection of the catalystdeterioration in accordance with the operating conditions of the engine,for example, the revolution speed (revolutions per minute) and loadthereof. These factors are considered also in the prior-art techniquedisclosed in the official gazette of Japanese Patent ApplicationLaid-open No. 91440/1990. Further, in a case where the air-fuel-ratiosensor located upstream of the catalyst has deteriorated, the detectionof the catalyst deterioration is affected depending upon the extent orcontent thereof. Therefore, it is sometimes necessary to inhibit acorrection for a detected result and the detection of the catalystdeterioration. In the prior-art technique disclosed in the officialgazette of Japanese Patent Application Laid-open No. 286160/1991,accordingly, the detection of the catalyst deterioration is inhibitedwhen the upstream air-fuel-ratio sensor has deteriorated.

Meanwhile, when the engine has misfired in the combustion strokethereof, oxygen in air flows into an exhaust pipe together with unburntgas. In consequence, the air-fuel-ratio sensor located upstream of thecatalyst generates a spike signal indicating the lean exhaust gas, orthe air-fuel-ratio sensor located downstream of the catalyst generates asignal indicating leaner exhaust gas than the actual one. This poses theproblem that the accuracy of the detection of the catalyst deteriorationlowers. Also in a case where a secondary air system disposed forintroducing the air into the exhaust pipe has failed, the problem of thelowering of the detection accuracy is involved for such a reason thatthe air-fuel-ratio feedback control does not proceed normally. Thesedrawbacks are not considered in any of the prior-art techniques.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a diagnostic equipmentfor an exhaust gas cleaning apparatus, in which the accuracy of thedetection of the deterioration of an air-fuel-ratio sensor as well as acatalyst is prevented from lowering even when the misfire of an engineand the failure of a secondary air system have taken place.

According to the present invention, when the misfire of an engine hasbeen detected, a correction is made for a deterioration index whichexpresses the extent of the deterioration of a catalyst or anair-fuel-ratio sensor and which is calculated in the detection of thecatalyst or sensor deterioration, or the decision of the deteriorationstate of the catalyst or air-fuel-ratio sensor is interrupted during thedetection of the misfire. Besides, in case of the failure of a secondaryair system, the correction or interruption stated above is similarlydone.

Incidentally, the correction may well be made only in a case wherefrequence in the misfire is comparatively low or where the failure ofthe secondary air system is comparatively light. The decision of thedeterioration state of the catalyst or air-fuel-ratio sensor may well beinterrupted in a case where the frequence in the misfire is high orwhere the failure of the secondary air system is heavy. On thisoccasion, the deterioration state of the catalyst or air-fuel-ratiosensor cannot be decided. In general, however, the misfire of the engineand the failure of the secondary air system do not often occur, and sucha construction is satisfactory in many uses. With this contrivance, evenwhen the misfire of the engine or the failure of the secondary airsystem has occurred, the detection of the deterioration of the catalystor air-fuel-ratio sensor is possible to some extent without lowering theaccuracy thereof.

The construction of the present invention will be described moreconcretely below.

In the first aspect of performance of the present invention, there isprovided a diagnostic equipment for an exhaust gas cleaning apparatus;the exhaust gas cleaning apparatus being directed toward an enginesystem furnished with an air-fuel-ratio controller which detects an airfuel ratio of exhaust gas emitted from an engine and which adjusts aquantity of fuel injection so as to hold the air fuel ratio of theexhaust gas at a predetermined value, and cleaning the exhaust gas bymeans of a catalyst; the diagnostic equipment comprising a firstair-fuel-ratio sensor which detects the air fuel ratio of the exhaustgas upstream of the catalyst; a second air-fuel-ratio sensor whichdetects the air fuel ratio of the exhaust gas downstream of thecatalyst; catalyst-deterioration-index calculation means for calculatinga catalyst deterioration index indicative of a deterioration state ofthe catalyst from output signals of the first air-fuel-ratio sensor andthe second air-fuel-ratio sensor; catalyst-deterioration decision meansendowed with a predetermined threshold value, for deciding thedeterioration state of the catalyst through a comparison between thethreshold value and the catalyst deterioration index; abnormalitydetection means for detecting any abnormality of the engine system asaffects the catalyst deterioration index; and at least one memberselected from the group consisting of catalyst-deterioration-indexcorrection means for correcting the catalyst deterioration index whenthe abnormality has been detected by the abnormality detection means,and catalyst-deterioration-decision interruption means for interruptingthe decision of the catalyst-deterioration decision means when theabnormality has been detected by the abnormality detection means.

Herein, the abnormality detection means may well comprise misfiredetection means for detecting a combustion state of the engine so as todetect occurrence of misfire of the engine.

The operation of the first aspect of performance of the presentinvention will be described.

The catalyst-deterioration-index calculation means calculates a catalystdeterioration index indicative of a deterioration state of the catalystfrom output signals of the first air-fuel-ratio sensor and the secondair-fuel-ratio sensor. The catalyst-deterioration decision means decidesthe deterioration state of the catalyst through a comparison between thethreshold value and the catalyst deterioration index. Thecatalyst-deterioration-index correction means corrects the catalystdeterioration index when the abnormality has been detected by theabnormality detection means, and the catalyst-deterioration-decisioninterruption means interrupts the decision of the catalyst-deteriorationdecision means when the abnormality has been detected by the abnormalitydetection means.

It is also allowed that the engine system includes a secondary airsystem which introduces air into a part of an exhaust pipe locatedbetween the engine and the catalyst, and that the abnormality detectionmeans comprises secondary-air-system failure detection means fordetecting a failure of the secondary air system.

In the second aspect of performance of the present invention, there isprovided a diagnostic equipment for an exhaust gas cleaning apparatus;the exhaust gas cleaning apparatus being directed toward an enginesystem furnished with an air-fuel-ratio controller which detects an airfuel ratio of exhaust gas emitted from an engine and which adjusts aquantity of fuel injection so as to hold the air fuel ratio of theexhaust gas at a predetermined value, and cleaning the exhaust gas bymeans of a catalyst; the diagnostic equipment comprising anair-fuel-ratio sensor which detects the air fuel ratio of the exhaustgas; sensor-deterioration-index calculation means for calculating anair-fuel-ratio-sensor-deterioration index indicative of a deteriorationstate of the air-fuel-ratio sensor from an output signal of theair-fuel-ratio sensor; sensor-deterioration decision means endowed witha predetermined threshold value, for deciding the deterioration state ofthe air-fuel-ratio sensor through a comparison between the thresholdvalue and the air-fuel-ratio-sensor-deterioration index; abnormalitydetection means for detecting any abnormality of the engine system asaffects the air-fuel-ratio-sensor-deterioration index; and at least onemember selected from the group consisting of sensor-deterioration-indexcorrection means for correcting the air-fuel-ratio-sensor-deteriorationindex when the abnormality has been detected by the abnormalitydetection means, and sensor-deterioration-decision interruption meansfor interrupting the decision of the sensor-deterioration decision meanswhen the abnormality has been detected by the abnormality detectionmeans.

Herein, the abnormality detection means may well comprise misfiredetection means for detecting a combustion state of the engine so as todetect occurrence of misfire of the engine.

It is also allowed that the engine system includes a secondary airsystem which introduces air into a part of an exhaust pipe locatedbetween the engine and the catalyst, and that the abnormality detectionmeans comprises secondary-air-system failure detection means fordetecting a failure of the secondary air system.

The operation of the second aspect of performance of the presentinvention will be described.

The sensor-deterioration-index calculation means calculates anair-fuel-ratio-sensor-deterioration index indicative of a deteriorationstate of the air-fuel-ratio sensor from an output signal of theair-fuel-ratio sensor. The sensor-deterioration decision means decidesthe deterioration state of the air-fuel-ratio sensor through acomparison between the threshold value and theair-fuel-ratio-sensor-deterioration index. Thesensor-deterioration-index correction means corrects theair-fuel-ratio-sensor-deterioration index when the abnormality has beendetected by the abnormality detection means. Thesensor-deterioration-decision interruption means interrupts the decisionof the sensor-deterioration decision means when the abnormality has beendetected by the abnormality detection means.

As stated above, according to the corresponding one of the aspects ofperformance of the present invention, the degree of deterioration of thecatalyst or the air-fuel-ratio sensor is not erroneously decided evenwhen the misfire of the engine and the failure or deterioration of thesecondary air system has occurred. It is therefore avoided to replacethe catalyst or the air-fuel-ratio sensor in spite of no deteriorationthereof, or to run the engine in spite of the deterioration of thecatalyst or the air-fuel-ratio sensor without the replacement thereof.Accordingly, the present invention serves to eliminate wasteful expensesand to prevent air pollution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the construction of one embodiment ofthe present invention;

FIG. 2 is a diagram showing examples of the output signals ofair-fuel-ratio sensors which are respectively mounted upstream anddownstream of a catalyst;

FIG. 3 is a graph for explaining the relationship between the degree ofcatalyst deterioration and the index of catalyst deterioration;

FIG. 4A is a graph showing the situation of the output signal of theupstream air-fuel-ratio sensor in the case where no misfire arises,while FIG. 4B is a graph showing the situation of the output signal ofthe upstream air-fuel-ratio sensor in the case where misfire arises;

FIG. 5 is a graph showing influences which the misfire exerts on thecatalyst deterioration index in a case where the downstreamair-fuel-ratio sensor is an O₂ -sensor;

FIG. 6 is a graph showing a correction coefficient and decisioninterruption regions which correspond to the case of FIG. 5;

FIG. 7 is a graph showing influences which the misfire exerts on thecatalyst deterioration index in a case where the downstreamair-fuel-ratio sensor is a so-called "UEGO sensor(universal exhaust gasoxygen sensor)";

FIG. 8 is a graph showing a correction coefficient and a decisioninterruption region which correspond to the case of FIG. 7;

FIG. 9 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is considered in correspondence with the O₂ -sensor in FIG. 5;

FIG. 10 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is considered in correspondence with the wide-region sensor in FIG.7;

FIG. 11 is a block diagram showing the construction of anotherembodiment of the present invention;

FIG. 12 is a graph showing influences which misfire exerts on an indexfor the deterioration of an air-fuel-ratio sensor which is mountedupstream of a catalyst in the embodiment depicted in FIG. 11;

FIG. 13 is a graph showing a correction coefficient and a decisioninterruption region which correspond to the case of FIG. 12; and

FIG. 14 is a graph showing a correction coefficient and a decisioninterruption region in the case where the leakage quantity of secondaryair is taken into consideration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, embodiments of the present invention will be described inconjunction with the accompanying drawings.

FIG. 1 is a block diagram showing the construction of a diagnosticequipment which is one embodiment of the present invention, togetherwith that of an engine system to which the diagnostic equipment isapplied.

First, the illustrated example of the engine system on which the presentinvention is premised will be outlined. However, an object to which thediagnostic equipment of the present invention is to be applied shall notbe restricted to the illustrated example.

The quantity Qa of air to be introduced into an engine 1 by suction ismeasured by an air flow meter 2. In addition, the revolution speed ornumber (revolutions per minute abbreviated to "r. p. m.") Ne of theengine 1 is measured by r. p. m. measurement means not shown.Air-fuel-ratio feedback control means 3 evaluates the basic injectionquantity F_(o) of fuel from the suction air quantity Qa and the r. p. m.Ne in accordance with Equation (1) given below:

    F.sub.o =kQa/Ne                                            (1)

k: coefficient

An air-fuel-ratio sensor 7 measures the air fuel ratio of exhaust gasemitted from the engine 1. As stated before, the expression"air-fuel-ratio sensor" in this specification shall cover both the "O₂-sensor" which detects the oxygen concentration in terms of a binaryvalue (and the output of which changes suddenly near the stoichiometricratio), and the so-called "UEGO sensor" which detects the oxygenconcentration linearly. The air-fuel-ratio feedback control means 3obtains a correction coefficient α in dependency on the output of theair-fuel-ratio sensor 7, and evaluates the injection quantity F bycorrecting the basic injection quantity F_(o) in accordance withEquation (2) given below:

    F=F.sub.o (1+α)                                      (2)

Further, the air-fuel-ratio feedback control means 3 applies to aninjector 4 a pulse signal whose width corresponds to the injectionquantity F. Thus, it subjects the quantity of fuel feed to a feedbackcontrol. Owing to such control operations, the air fuel ratio of mixtureis held at or near the stoichiometric ratio. A catalyst 5 for decreasingnoxious components contained in the exhaust gas, oxidizes unburnt gascomponents (hydrocarbons abbreviated to "HC") and carbon monoxide (CO)and simultaneously deoxidizes nitrogen oxides (NO_(x)). Such a catalystis called a "ternary catalyst". The air fuel ratio needs to be held ator near the stoichiometric ratio in order that the oxidizing anddeoxidizing reactions based on the ternary catalyst may be effected atthe same time. In turn, the air-fuel-ratio feedback control needs to beprecisely executed for keeping the stoichiometric ratio.

A secondary air system 6 introduces air into an exhaust pipe 100 bymeans of a pump 60 in the operating condition of the engine 1 (forexample, at the start of the engine 1) as requires the establishment ofa state in which the mixture contains the fuel in excess of thestoichiometric ratio (that is, the mixture is "rich"). Thus, thesecondary air system 6 functions to burn and decrease the surplus HCemitted into the exhaust pipe 100. Although not clearly seen from thedrawing, the pump 60 is constructed so as to be operable in associationwith the air-fuel-ratio feedback control means 3, etc.

Next, the diagnostic equipment of this embodiment will be described.

As illustrated in FIG. 1, the diagnostic equipment of this embodiment isconstructed comprising misfire detection means 9, secondary-air-systemfailure detection means 10, catalyst-deterioration-index calculationmeans 11, catalyst-deterioration decision means 12,catalyst-deterioration-index correction means 13 andcatalyst-deterioration-decision interruption means 14.

The catalyst-deterioration-index calculation means 11 serves to detectthe deterioration state of the catalyst 5. Thecatalyst-deterioration-index calculation means 11 in this embodimentdetects the deterioration of the catalyst 5 by utilizing the correlationbetween the signals of the air-fuel-ratio sensor 7 located upstream ofthe catalyst 5 and an air-fuel-ratio sensor 8 located downstream of thesame 5. More specifically, in the state in which the catalyst 5undergoes no deterioration, the output signal of the downstreamair-fuel-ratio sensor 8 does not fluctuate similarly to that of theupstream air-fuel-ratio sensor 7. However, as the catalyst 5deteriorates, the output signal of the downstream air-fuel-ratio sensor8 comes to exhibit a fluctuation similar to that of the output signal ofthe upstream air-fuel-ratio sensor 7. These facts are utilized for thedetection of the catalyst deterioration. Such a method of detecting thecatalyst deterioration will be outlined below.

The catalyst-deterioration-index calculation means 11 first measures theoutput signals of the air-fuel-ratio sensors 7 and 8 synchronously.Subsequently, D. C. (direct-current) components which disturb thedeterioration detection are removed from the measured signals by the useof high-pass filters. FIG. 2 illustrates examples of these signals. Inthe figure, symbol x denotes the signal obtained by removing the D. C.component from the output signal of the upstream air-fuel-ratio sensor7, while symbol y denotes the signal obtained by removing the D. C.component from the output signal of the downstream air-fuel-ratio sensor8.

Besides, the catalyst-deterioration-index calculation means 11calculates the auto-correlation function φxx (Equation (3) given below)of the signal x and the cross-correlation function φxy (Equation (4)) ofthe signals x and y:

    φxx(τ)=∫x(t)x(t-τ)dt                      (3)

t: time

τ: phase

    φxy(τ)=∫x(t)y(t-τ)dt                      (4)

t: time

τ: phase

Since the auto-correlation function φxx(τ) assumes the maximum valueφxx(0) for τ=0, the relationship of the following equation (5) holds:

    (φxx).sub.max =φxx(0)                              (5)

Further, the maximum value of the cross-correlation function φxy(τ) isfound by varying the phase τ within the integral section of thisfunction φxy(τ). Assuming that the maximum value is assumed for τ=τ_(o),the following equation (6) is obtained:

    (φxy).sub.max =φxy(τ.sub.o)                    (6)

The catalyst deterioration index Φc is calculated from these values inaccordance with the following equation (7):

    Φc=(φxy).sub.max /(φxx).sub.max                (7)

FIG. 3 illustrates the relationship between a formula φxy(τ)/(φxx)_(max)and the degree of catalyst deterioration. In a case where the degree ofthe deterioration of the catalyst 5 is high (the catalyst 5 hasdeteriorated heavily), the degree of the correlation between the outputsignal of the upstream air-fuel-ratio sensor 7 and that of thedownstream air-fuel-ratio sensor 8 is high, and a high peak (maximumvalue) is demonstrated. On the other hand, in a case where the degree ofthe deterioration of the catalyst 5 is low (the catalyst 5 hasdeteriorated lightly), the degree of the correlation between both theoutput signals is low, and only a low peak (maximum value) isdemonstrated. Accordingly, the degree of the deterioration of thecatalyst 5 can be detected in accordance with the magnitude of thecatalyst deterioration index Φc.

Incidentally, the method of detecting the deterioration degree statedhere has already been proposed in Japanese Patent Laid-Open(KOKAI)No.171924/1993.

The method of detecting the catalyst deterioration shall not berestricted to the aforementioned method. It is also allowed to employ,for example, a method wherein the catalyst deterioration is detectedfrom the fluctuation widths, phase difference, frequencies or the likeof the output signals of the air-fuel-ratio sensors 7 and 8. A largenumber of other examples have also been known, and any of these methodsmay well be employed. It is necessary, however, to alter a correctionmethod etc. which will be explained below, in correspondence with theemployed detection method.

The catalyst-deterioration decision means 12 checks the deteriorationstate of the catalyst 5 found by the catalyst-deterioration-indexcalculation means 11, thereby deciding whether or not the exhaust gascleaning apparatus of the engine system is faulty. In this embodiment,the decision is rendered by comparing the catalyst deterioration indexΦc with a predetermined value set beforehand (the predetermined valuecorresponds to a "threshold value" in the appended claims). By way ofexample, when the catalyst deterioration index Φc is greater than thepredetermined value, the fault of the exhaust gas cleaning apparatus(namely, the failure of the catalyst 5) is decided. In the case of thefault, the driver of a vehicle on which the engine system is installedis warned by lighting up an alarm lamp not shown.

The misfire detection means 9 functions to detect the presence orabsence of misfire every combustion stroke proceeding in the cylinder ofthe engine 1. A method of detecting the misfire is, for example, onedisclosed in the official gazette of Japanese Patent ApplicationLaid-open No. 206342/1991 wherein the misfire is detected from thefluctuation of the revolution speed (r. p. m.) of an engine. This methoddetects an r. p. m. fluctuation waveform or a combustion state duringthe combustion stroke of the engine. Another method of detecting thecombustion state on the basis of the r. p. m. fluctuation is disclosedin U.S. Pat. No. 4,627,399. Still another method detects the combustionstate from a combustion pressure, a temperature or/and the like in thecombustion chamber of the engine, or from the pulsation of the pressureof exhaust gas or the temperature of the exhaust gas. Further, therehave been known a method wherein the combustion state is detected froman ionic current which flows across an ion gap formed in the combustionchamber, as disclosed in U.S. Pat. No. 4,648,367, a method wherein it isdetected by measuring combustion light in the combustion chamber, and amethod wherein it is detected from the waveform of current flowingthrough an ignition coil, or the like. The misfire detection means 9 inthis embodiment may employ any of such numerous known methods.

The secondary-air-system failure detection means 10 functions to detectthe failure and deterioration of the secondary air system 6. This means10 is realized by, for example, a method wherein a flow meter isdisposed midway of the air passage of the secondary air system 6 so asto detect the actual flow rate of air and wherein the failure ordeterioration is detected on the basis of the difference between thedetected actual flow rate and a flow rate estimated from the controlledvariable (voltage or current value) of the pump 60. Another method isbased on the correction coefficient a explained before. Morespecifically, in the state in which the secondary air system 6 isoperated, the air-fuel-ratio sensor 7 detects oxygen in a largerquantity. Therefore, when the air-fuel-ratio feedback control explainedbefore is performed as it is, the correction coefficient α enlarges soas to increase the quantity of feed fuel. By exploiting this fact, themethod decides the flow of no secondary air and detects the failure ofthe secondary air system 6 in such a case where the correctioncoefficient α does not enlarge in spite of the actuation of, e. g., thepump 60. The secondary-air-system failure detection means 10 may employany of such numerous known methods.

The catalyst-deterioration-index correction means 13 corrects thecatalyst deterioration index Φc when the misfire of the engine 1 or thefailure or deterioration of the secondary air system 6 has been detectedby the misfire detection means 9 or by the secondary-air-system failuredetection means 10. Then, the correction means 13 delivers the correctedcatalyst deterioration index to the catalyst-deterioration decisionmeans 12. Incidentally, the correction method will be described latertogether with the operation of this embodiment.

The catalyst-deterioration-decision interruption means 14 is endowedwith a predetermined frequence or extent which concerns the misfire ofthe engine 1 or the failure and deterioration of the secondary airsystem 6 and which serves as a criterion for the interruption of thecatalyst-deterioration decision. Thus, when the misfire or the failureor deterioration detected by the misfire detection means 9 or thesecondary-air-system failure detection means 10 exceeds thepredetermined frequence or extent, the interruption means 14 generates asignal for interrupting the decision of the deterioration of thecatalyst 5 by the catalyst-deterioration decision means 12. By the way,the frequence or extent may well be altered depending upon temperatureetc. as will be explained later.

There will be described the operation of this embodiment in the casewhere the engine 1 misfires.

FIGS. 4A and 4B illustrate examples of the output signals of theair-fuel-ratio sensor 7 located upstream of the catalyst 5, in anon-misfiring condition and a misfiring condition, respectively.

When the misfire has occurred, the unburnt HC and the air flow into theexhaust pipe 100. Therefore, a spike signal S4 which indicates that theexhaust gas is lean (in other words, the exhaust gas contains the largerquantity of oxygen) appears in synchronism with the misfire. In such amisfiring condition, the reactions of oxidizing the unburnt HC proceedtogether with the ordinary cleaning reactions within the catalyst 5.Nevertheless, the HC and oxygen components which have not reacted flowdownstream of the catalyst 5. In consequence, the downstreamair-fuel-ratio sensor 8 is affected as stated below.

In the case where the air-fuel-ratio sensor 8 is the O₂ -sensor whichproduces the binary output, the output signal shifts to the side thereofindicating the leaner exhaust gas and also has its amplitude decreased.As illustrated in FIG. 5, therefore, the catalyst deterioration index Φcchanges in correspondence with the frequence in the misfire and becomesa smaller value. That is, the degree of the catalyst deterioration isestimated to be lower than the actual one. In thecatalyst-deterioration-index correction means 13, accordingly, acorrection which enlarges the catalyst deterioration index Φc inaccordance with the frequence in the misfire may be performed bymultiplying the index Φc by a correction coefficient Kc as illustratedin FIG. 6 by way of example. Alternatively, it is allowed to perform acorrection in which the predetermined value to be compared with thecatalyst deterioration index Φc in the catalyst-deterioration decisionmeans 12 is made smaller in accordance with the frequence in the misfirecontrariwise to the above. In such a case where one cylinder continuesto misfire in the engine 1 of, e. g., six cylinders, oxygen and HC whichought to react in proper quantities by the combustion do not entirelyreact within the exhaust pipe 100 as well as the catalyst 5, and hence,the larger quantity of oxygen flows even to the lower stream part of theexhaust pipe 100 with respect to the catalyst 5. In consequence, thedownstream air-fuel-ratio sensor 8 delivers the output signal whichindicates that the air fuel ratio is greater than the stoichiometricratio (in other words, the exhaust gas is leaner than one of thestoichiometric ratio), irrespective of the degree of the deteriorationof the catalyst 5. Moreover, the signal becomes a fixed value (thesignal of substantially null amplitude). As a result, the catalystdeterioration index Φc assumes a fixed value irrespective of the degreeof the catalyst deterioration and cannot be corrected. Accordingly, in acase where the occurring frequence in the misfire is lower than apredetermined value, the catalyst deterioration index Φc shouldpreferably be corrected in accordance with the pertinent frequence. Onthe other hand, in a case where the frequence is higher than thepredetermined value, the decision of the catalyst deterioration shouldpreferably be interrupted while the misfire is occurring or thefrequence is in excess of the predetermined value.

The magnitude of the correction and the misfire frequence permitting thecorrection, change depending upon the temperature of the catalyst 5, theload of the engine 1, etc. This is based on such a reason that thevelocities of the oxidizing reactions of the HC differ depending uponthe temperature of the catalyst 5 (the reaction velocities are higher asthe temperature is higher within a temperature range which the catalyst5 can usually assume). It is therefore favorable that the correctioncoefficient Kc and the misfire frequence for interrupting the catalystdeterioration decision are altered as exemplified in FIG. 6, dependingupon the temperature of the catalyst 5 and the operating conditions ofthe engine 1. Many of the actual misfiring states of the engine 1 aresuch that one or more cylinders fail to ignite substantiallycontinuously on account of, e. g., the trouble of an ignition system. Inthis case, the misfire at the frequence not permitting the correction ofthe catalyst deterioration index Φc is detected at one stroke.Accordingly, it is often satisfactory that, when the misfire hasoccurred at the frequence affecting the detection of the catalystdeterioration, the decision of the catalyst deterioration is interruptedimmediately without performing the index correction. In this case, inmany of the known examples, the misfire detection means 9 gives warningto the driver against the occurrence of the misfire. In practical use,therefore, it hardly poses any problem that the decision of the catalystdeterioration is restarted after the ignition system, for example, hasbeen repaired on the basis of the warning.

Meanwhile, in the case where the downstream air-fuel-ratio sensor 8 isthe so-called "UEGO sensor", it delivers the output signal indicatingthe lean exhaust gas in substantial synchronism with the spike outputsignal of the upstream air-fuel-ratio sensor 7 in the misfiringcondition (refer to FIG. 4B). As illustrated in FIG. 7, therefore, whenthe frequence in the misfire is low, the catalyst deterioration index Φcbecomes a value which is greater though slightly. When the frequence inthe misfire is somewhat high, the catalyst 5 does not react normally inspite of no deterioration thereof, so that the air-fuel-ratio sensors 7and 8 deliver signal waveforms being closely similar to each other, andthe catalyst deterioration index Φc demonstrates a great value. It isaccordingly recommended that, as illustrated in FIG. 8 by way ofexample, the correction of multiplying the catalyst deterioration indexΦc by the coefficient Kc which makes this index Φc smaller is performedwhen the misfire frequence is low, while the decision of the catalystdeterioration is inhibited when the misfire frequence is high.

As in the foregoing case of the O₂ -sensor, it is often satisfactorythat, when the misfire has occurred at the frequence affecting thedetection of the catalyst deterioration, the decision of the catalystdeterioration is interrupted immediately without performing the indexcorrection. Further, on this occasion, the catalyst deterioration indexΦc becomes a greater value in the decision of the catalystdeterioration, and hence, the catalyst 5 undergoing no deteriorationmight have been erroneously decided as undergoing the deterioration. Itis therefore favorable to discard the last result of the decision of thecatalyst deterioration.

The diagnostic equipment may well be so constructed that, unlike to theabove, only the correction by the catalyst-deterioration-index (Φc)correction means 13 is done without disposing thecatalyst-deterioration-decision interruption means 14.

Next, the operation of this embodiment will be described concerning thecase where the secondary air system 6 has failed or deteriorated.

As stated before, the secondary air system 6 serves to introduce thesecondary air into the exhaust pipe 100 by means of the pump 60 and toburn and decrease the surplus HC emitted into the exhaust pipe 100,generally in the operating condition in which the mixture must be made"rich" or set at a smaller air fuel ratio (for example, at the start ofthe engine 1). Usually, the secondary air system 6 is not in operationduring the air-fuel-ratio feedback control. In the catalystdeterioration detecting method of this embodiment, accordingly, aspecial problem is such a failure that the secondary air flows out ofthe secondary air system 6 during the sampling of the data by theair-fuel-ratio sensors 7 and 8.

Influences in the case of the occurrence of such a failure differdepending upon the positional relationship between the air-fuel-ratiosensor 7 and the inlet 62 of the secondary air system 6 to the exhaustpipe 100, and so forth.

In the case where the air-fuel-ratio sensor 7 is located on the lowerstream side of the exhaust pipe 100 with respect to the inlet 62 (asillustrated in FIG. 1), it detects oxygen which has flowed in from thesecondary air system 6, thereby deciding that the exhaust gas is "lean".Consequently, the air-fuel-ratio feedback control means 3 performs thefeedback control so as to increase the feed fuel (namely, to bring themixture to the "rich" side thereof). As a result, the quantity of thefuel enlarges in excess of an amount which corresponds to the oxygenhaving flowed in from the secondary air system 6. Eventually, theexhaust gas becomes "rich" at the position of the catalyst S. Therefore,the output signal of the air-fuel-ratio sensor 8 mounted downstream ofthe catalyst 5 shifts to the "rich" side thereof.

In this regard, in the case where the downstream air-fuel-ratio sensor 8is the O₂ -sensor of the type which detects the air fuel ratio in termsof the binary value, the detected air fuel ratio deviates from the airfuel ratio of the output variation thereof, and the amplitude of theoutput waveform thereof decreases. As in the case of FIG. 5, therefore,the catalyst deterioration index Φc assumes a smaller value inaccordance with the leakage quantity of the secondary air. When theleakage quantity of the secondary air enlarges more, the output signalof the air-fuel-ratio sensor 8 falls into a fixed state ("rich" state).Therefore, the catalyst deterioration index Φc becomes substantiallynull and can no longer be corrected. It is accordingly recommended that,as illustrated in FIG. 9, the catalyst deterioration index Φc ismultiplied by the coefficient Kc for its correction when the leakagequantity of the secondary air is small, while the detection of thecatalyst deterioration is interrupted when the leakage quantity islarge.

In general, the secondary-air-system failure detection means 10 becomescostly for the precise detection of the leakage quantity, so that theaccuracy thereof is not very high. Therefore, it is sometimes the casethat the decision of the catalyst deterioration is already impossiblewhen the failure of the secondary air system 6 has been decided.Accordingly, the decision of the deterioration may well be interruptedat one stroke without performing the index correction. Alternatively,the diagnostic equipment may well be so constructed that, unlike to theabove, only the correction by the catalyst-deterioration-index (Φc)correction means 13 is done without disposing thecatalyst-deterioration-decision interruption means 14.

On the other hand, in the case where the downstream air-fuel-ratiosensor 8 is the UEGO sensor, the leakage of the secondary air incurs thefluctuation of the air fuel ratio exceeding the range thereof in whichthe catalyst 5 can efficiently carry out the oxidizing and deoxidizingreactions, and it enlarges the amplitude of the output signal of theair-fuel-ratio sensor 8. As a result, the catalyst deterioration indexΦc assumes a greater value in accordance with the leakage quantity ofthe secondary air. When the leakage quantity of the secondary airenlarges more, the output signal of the air-fuel-ratio sensor 8 exceedsthe full-scale value thereof irrespective of the degree of thedeterioration of the catalyst 5, and the catalyst deterioration index Φccan no longer be corrected. It is accordingly recommended that, asillustrated in FIG. 10, the catalyst deterioration index Φc ismultiplied by the coefficient Kc for its correction when the leakagequantity from the secondary air system 6 is small, while the detectionof the catalyst deterioration is interrupted when the leakage quantityis large. As in the foregoing case, it is also allowed herein that, whenthe failure of the secondary air system 6 has been decided, the decisionof the catalyst deterioration is interrupted at one stroke withoutperforming the index correction. Further, on this occasion, the catalystdeterioration index Φc becomes the greater value in the decision of thecatalyst deterioration, and hence, the catalyst 5 undergoing nodeterioration might have been erroneously decided as undergoing thedeterioration. It is therefore favorable to discard the last result ofthe decision of the catalyst deterioration. The diagnostic equipment maywell be so constructed that, unlike to the above, only the correction bythe catalyst-deterioration-index (Φc) correction means 13 is donewithout disposing the catalyst-deterioration-decision interruption means14.

As thus far described, in the case where the leakage quantity of thesecondary air is small, the catalyst deterioration index Φc is correctedby the catalyst-deterioration-index correction means 13. On thisoccasion, the directions of the corrections are the opposite incorrespondence with the sorts of the air-fuel-ratio sensor 8. On theother hand, in the case where the leakage quantity of the secondary airis large, the decision of the catalyst deterioration should preferablybe interrupted by the catalyst-deterioration-decision interruption means14.

The leakage quantity of the secondary air permitting the correctionchanges depending upon the temperature of the catalyst 5, etc. It istherefore favorable that the magnitude of the correction and the leakageair quantity for interrupting the catalyst deterioration decision arealtered depending upon the temperature of the catalyst 5 or/and theoperating conditions of the engine 1. In general, likewise to thecorrection ascribable to the misfire, the correction magnitude is madesmaller as the load of the engine 1 is heavier. In addition, the leakageair quantity for interrupting the decision is altered to the sidethereof on which the leakage quantity is larger.

Now, a diagnostic equipment for deciding the deterioration of anair-fuel-ratio sensor will be described as the second embodiment of thepresent invention.

FIG. 11 generally illustrates the construction of the diagnosticequipment in this embodiment, together with that of an engine system towhich the diagnostic equipment is applied. An engine 1, an air flowmeter 2, air-fuel-ratio feedback control means 3, an injector 4, anair-fuel-ratio sensor 7, etc. are the same as those described in thefirst embodiment, respectively.

The diagnostic equipment is constructed comprising misfire detectionmeans 9, secondary-air-system failure detection means 10,sensor-deterioration-index calculation means 21, sensor-deteriorationdecision means 22, sensor-deterioration-index correction means 23 andsensor-deterioration-decision interruption means 24.

The sensor-deterioration-index calculation means 21 serves to detect thedeterioration state of the air-fuel-ratio sensor 7. In this embodiment,the detection of the deterioration state is effected using theauto-correlation function of the output signal of the air-fuel-ratiosensor 7. More specifically, the sensor deterioration-index calculationmeans 21 calculates the auto-correlation function φxx(0) on the basis ofthe signal x (refer to FIG. 2) obtained by removing the D. C. componentfrom the output signal of the air-fuel-ratio sensor 7, in the samemanner as in the foregoing case of the catalyst-deterioration-indexcalculation means 11. In case of adopting the function φxx(0) as adeterioration index Φsr which expresses the degree of deterioration ofthe responsivity of the air-fuel-ratio sensor 7, the deterioration index(sr assumes a greater value when the air-fuel-ratio sensor 7 undergoesno deterioration, and it assumes a smaller value when the air-fuel-ratiosensor 7 undergoes the deterioration.

The method of detecting the deterioration of the air-fuel-ratio sensor 7is not restricted to the above one. There have been known the otherexamples of detection methods each of which is based on the differentialvalue of the output signal of the air-fuel-ratio sensor 7, a responsetime period required for the output signal to change to the amount of apredetermined voltage, or the frequency of the output signal. Any ofthese methods may well be employed. Merely, a correction method etc.which will be explained below come to differ in correspondence with theemployed detection method.

The sensor-deterioration decision means 22 compares the sensordeterioration index Φsr with a predetermined value (the predeterminedvalue corresponds to a "threshold value" in the appended claims),thereby deciding the degree of the deterioration of the air-fuel-ratiosensor 7. When the sensor deterioration index Φsr is greater than thepredetermined value, the failure of the sensor 7 is decided. In the caseof the failure, the driver of a vehicle on which the engine system isinstalled is warned by, for example, lighting up an alarm lamp notshown.

The misfire detection means 9 and the secondary-air-system failuredetection means 10 are similar to those in the first embodiment,respectively.

The sensor-deterioration-index correction means 23 corrects the sensordeterioration index Φsr when the misfire of the engine 1 or the failureor deterioration of a secondary air system 6 has been detected by themisfire detection means 9 or by the secondary-air-system failuredetection means 10. Then, the correction means 23 delivers the correctedsensor deterioration index to the sensor-deterioration decision means22.

The sensor-deterioration-decision interruption means 24 is endowed witha predetermined frequence or extent which concerns the misfire of theengine 1 or the failure of the secondary air system 6 and which servesas a criterion for the interruption of the sensor-deteriorationdecision. Thus, when the misfire or the failure or deteriorationdetected by the misfire detection means 9 or the secondary-air-systemfailure detection means 10 exceeds the predetermined frequence orextent, the interruption means 24 generates a signal for interruptingthe decision of the deterioration of the air-fuel-ratio sensor 7 by thesensor-deterioration decision means 22.

There will be described the operation of this embodiment in the casewhere the engine 1 misfires.

When the misfire occurs, a spike "lean" signal (S4 in FIG. 4B) appears.Then, the sensor deterioration index Φsr assumes a greater value,depending upon the frequence in the occurrence of the misfire (refer toFIG. 12). That is, the responsivity of the air-fuel-ratio sensor 7 isestimated to be better than the actual one, and the degree of the sensordeterioration is decided to be lower. This holds true without regard tothe sorts of the air-fuel-ratio sensor 7. In thesensor-deterioration-index correction means 23, accordingly, acorrection which makes the sensor deterioration index Φsr smaller isperformed by multiplying the index Φsr by a correction coefficient Ksrwhich is determined in accordance with the frequence in the misfire(refer to FIG. 13). When the frequence in the misfire is too high, thesensor deterioration index Φsr disperses greatly. Therefore, thedecision of the deterioration of the air-fuel-ratio sensor 7 isinterrupted by the sensor-deterioration-decision interruption means 24.

Next, the operation of this embodiment will be described concerning thecase where the secondary air system 6 has failed.

As in the case of detecting the deterioration of a catalyst 5, a specialproblem is such a failure that the secondary air leaks through thesecondary air system 6 during the sampling of the data by theair-fuel-ratio sensor 7. Influences in the case of the occurrence ofsuch a failure differ depending upon, e. g., the mixed state of exhaustgas emitted from the engine 1 and the leakage air flowing in from thesecondary air system 6, in that part of an exhaust pipe 100 whichextends between the air-fuel-ratio sensor 7 and the inlet 62 of thesystem 6 to the exhaust pipe 100.

In this regard, in the case where the air-fuel-ratio sensor 7 is the O₂-sensor of the type which detects the air fuel ratio in terms of thebinary value, a signal on the "rich" side thereof becomes difficult ofappearing on account of the secondary air (oxygen) leaking in from theinlet 62. Therefore, the amplitude of the output waveform of theair-fuel-ratio sensor 7 decreases, and the sensor deterioration indexΦsr assumes a smaller value, in accordance with the leakage quantity ofthe secondary air. When the leakage quantity of the secondary airenlarges more, the sensor deterioration index Φsr becomes substantiallynull and can no longer be corrected. As illustrated in FIG. 14,accordingly, the sensor deterioration index Φsr is multiplied by thecoefficient Ksr for its correction when the leakage quantity of thesecondary air is small, while the detection of the sensor deteriorationis interrupted when the leakage quantity is large.

On the other hand, in the case where the air-fuel-ratio sensor 7 is theUEGO sensor, the sensor deterioration index Φsr assumes a smaller valuesubject to the leakage quantity which is small. When the leakagequantity enlarges more, the output of the air-fuel-ratio sensor 7becomes unstable, and the decision of the sensor deterioration can nolonger be rendered. Accordingly, the same correction anddeterioration-decision interruption as in FIG. 14 are done.

As stated before, in general, the accuracy of the secondary-air-systemfailure detection means 10 is not very high. Therefore, it is sometimesthe case that the decision of the deterioration of the air-fuel-ratiosensor 7 is already impossible when the failure of the secondary airsystem 6 has been decided. Accordingly, the decision of thedeterioration may well be interrupted at one stroke in the case wherethe failure of the secondary air system 6 has been decided.Alternatively, the diagnostic equipment may well be so constructed that,unlike to the above, only the correction by thesensor-deterioration-index correction means 23 is done without disposingthe sensor-deterioration-decision interruption means 24.

Although the decision of the deterioration of the air-fuel-ratio sensor7 concerning the responsivity thereof has been explained above, thedeterioration of the air-fuel-ratio sensor 7 concerning the outputvoltage thereof may well be decided as another example. In this example,the deterioration is detected by measuring the output of theair-fuel-ratio sensor 7 with the air fuel ratio of the exhaust gas heldat a predetermined value. Herein, the air fuel ratio fluctuates due tothe misfire of the engine 1 or the failure of the secondary air system6, so that the correction of the deterioration index and theinterruption of the deterioration decision are required.

By way of example, in case of deciding the output voltage of the "rich"side signal of the air-fuel-ratio sensor 7 under the condition that theair fuel ratio is held at the predetermined value in the "rich" state ofthe exhaust gas, the air-fuel-ratio sensor 7 generates a "lean" signalwhen the secondary air is leaking through the secondary air system 6. Asa result, the deterioration of the air-fuel-ratio sensor 7 iserroneously decided. Besides, a similar situation takes place when theengine 1 misfires. In case of a low frequence in the misfire, theair-fuel-ratio sensor 7 delivers a "lean" spike signal (S4 in FIG. 4B).On the other hand, in case of a high frequence in the misfire, theoutput signal of the binary air-fuel-ratio sensor 7 demonstrates a fixedvalue ("lean" state), or the output signal of the UEGO sensor 7disperses greatly. Accordingly, the present invention is also applicableto such a deterioration decision.

In addition, although the detection of the deterioration of theair-fuel-ratio sensor 7 mounted upstream of the catalyst 5 has beenreferred to in this embodiment, the correction of the deteriorationindex and the interruption of the deterioration decision need to be donein correspondence with the misfire of the engine 1 and the failure ofthe secondary air system 6, also in the detections of the deteriorationsof an air-fuel-ratio sensor mounted downstream of the catalyst 5. Thepresent invention is also applicable to the deterioration detections.

As thus far explained, the correction of the sensor deterioration indexKsr and the interruption of the sensor deterioration decision need to bedone in correspondence with the misfire of the engine 1 and the failureof the secondary air system 6, also in the detection of thedeterioration of the air-fuel-ratio sensor, so that the presentinvention is also applicable to the deterioration detection.

Further, although the case of correcting the catalyst or sensordeterioration index has been described in the above, it is substantiallythe same to correct the threshold value with which the catalyst orsensor deterioration index is compared in order to decide thedeterioration.

What is claimed is:
 1. Diagnostic equipment for an exhaust gas cleaning apparatus;said exhaust gas cleaning apparatus being directed toward an engine system furnished with an air-fuel-ratio controller which detects an air fuel ratio of exhaust gas emitted from an engine and which adjusts a quantity of fuel injection so as to hold the air fuel ratio of the exhaust gas at a predetermined value, and cleaning said exhaust gas by means of a catalyst; said diagnostic equipment, comprising:an air-fuel-ratio sensor which detects the air fuel ratio of the exhaust gas; sensor-deterioration-index calculation means for calculating an air-fuel-ratio-sensor-deterioration index indicative of a deterioration state of said air-fuel-ratio sensor from an output signal of said air-fuel-ratio sensor; sensor-deterioration decision means endowed with a predetermined threshold value, for deciding the deterioration state of said air-fuel-ratio sensor through a comparison between the threshold value and the air-fuel-ratio-sensor-deterioration index; abnormality detection means for detecting any abnormality of said engine system as affects said air-fuel-ratio-sensor-deterioration index; and sensor-deterioration-index correction means for correcting said air-fuel-ratio-sensor-deterioration index when the abnormality has been detected by said abnormality detection means.
 2. The diagnostic equipment for an exhaust gas cleaning apparatus as defined in claim 1, further comprising sensor-deterioration-decision interruption means for interrupting the decision of said sensor-deterioration decision means when a frequency of abnormality detected by said abnormality detection means exceeds a predetermined frequency.
 3. Diagnostic equipment for an exhaust gas cleaning apparatus;said exhaust gas cleaning apparatus being directed toward an engine system having a secondary air system which introduces air into a part of an exhaust pipe located between said engine and a catalyst, and furnished with an air-fuel-ratio controller which detects an air fuel ratio of exhaust gas emitted from an engine and which adjusts a quantity of fuel injection so as to hold the air fuel ratio of the exhaust gas at a predetermined value, and cleaning said exhaust gas by means of a catalyst; said diagnostic equipment, comprising:an air-fuel-ratio sensor which detects the air fuel ratio of the exhaust gas; sensor-deterioration-index calculation means for calculating an air-fuel-ratio-sensor-deterioration index indicative of a deterioration state of said air-fuel-ratio sensor from an output signal of said air-fuel-ratio sensor; sensor-deterioration decision means endowed with a predetermined threshold value, for deciding the deterioration state of said air-fuel-ratio sensor through a comparison between the threshold value and the air-fuel-ratio-sensor-deterioration index; abnormality detection means comprising secondary-air-system failure detection means for detecting a failure of said secondary-air-system as affects said air-fuel-ratio-sensor-deterioration index; and at least one of sensor-deterioration-index correction means for correcting said air-fuel-ratio-sensor-deterioration index when the abnormality has been detected by said abnormality detection means, and sensor-deterioration-decision interruption means for interrupting the decision of said sensor-deterioration decision means when the abnormality has been detected by said abnormality detection means. 